Despite clinical improvements, immune dysfunction and infection continue to be common in trauma victims. Innate and adaptive immune systems collaborate to protect against infection and they rely on each other for amplification for adequate host defense and regulation to prevent excessive activation and immunopathology. Post-injury neutrophils delay apoptosis to maintain host defense and prime adaptive immune responses during which time they can differentiate and upregulate the expression of HLA class II and several T cell co-inhibitory molecules. These co-inhibitory molecules suppress both T cell and neutrophil activation as a result of cell-cell receptor binding. The project's objective is to elucidate the role of the post-injury neutrophil in regulating T cell function. We hypothesize that post-injury neutrophils increase expression of co-inhibitory molecules that contribute to the inhibition of T cell and neutrophil responses. We propose that post-injury patients who demonstrate up-regulation in the neutrophil expression of MHC class II and co-inhibitory molecules will be at risk for immunosuppression and infection. We will study the role of post-injury neutrophil expressed co- inhibitory molecules in the modulation of T cell and neutrophil activation in vitro. The specific aims of the proposed research are to: 1) establish the expression pattern and functional effect of co-inhibitory molecules ILT4 and PD-L1 on donor and post-injury neutrophils, 2) demonstrate inhibition of T cell activation, including HLA-G+ Treg, Th17 cells, and CD4+CD25+ Treg subpopulations by ILT4+PD-L1+ neutrophils from trauma patients, 3) demonstrate the specificity of the inhibition by ILT4 and PD-L1 using RNA silencing of their expression in differentiated HL-60 cells. These experiments may 1) reveal a novel functional role of neutrophils in post-injury adaptive immune dysfunction, and 2) suggest new targets for modulation of the immuno-inflammatory response post-injury to minimize the risk of infection. Page 1 of 1 PUBLIC HEALTH RELEVANCE: Infection, sepsis, and organ failure continue to be the most common cause of mortality in trauma victims who survive more than 48 hours despite improvements in surgical management and critical care. Post-injury patients at highest risk for infectious morbidity appear immunosuppressed. We propose that patients who demonstrate a marked up-regulation in the neutrophil expression of inhibitory surface molecules will be at risk for immunosuppression and infection. These confirming experiments may 1) reveal a novel functional role of neutrophil inhibitory molecules in post-injury immune dysfunction, and 2) suggest new targets for modulation of the immuno-inflammatory response to reduce the incidence of post-injury infection and sepsis.